1. Field of the Invention
The present invention relates to a load current supply circuit having a current sensing function, in which the load current is supplied by a power transistor and the load current is sensed by a sensing circuit, and more particularly to a circuit constituting a current mirror circuit which includes a current sensing function.
2. Description of Related Art
A load current supply circuit having a current sensing function is disclosed, for example, in U.S. Pat. No. 4,553,084. In the circuit disclosed in this U.S. patent, a current mirror circuit is formed by connecting a power MOS transistor and a sense MOS transistor in parallel, and the current flowing through the power transistor is sensed by current flowing through the sense transistor. Respective current flowing through the power transistor and the sense transistor is proportional to the number of transistor cells included in the power transistor and the sense transistor, respectively. For example, assuming the number of cells in the power transistor is 10,000 and that in the sense transistor is 10, the current flowing through the power transistor is 1000 times the current flowing through the sense transistor, i.e., a current mirror ratio is 1000.
In such a current mirror circuit, an ON-resistance of the power transistor is much lower than that of the sense transistor. Therefore, a ratio of a wiring resistance to the ON-resistance in the power transistor circuit is much different from that in the sense transistor circuit. As exemplified in FIG. 4, assuming the ON-resistance of the power transistor is 110 m.OMEGA., the wiring resistance of the power transistor is 40 M.OMEGA., the ON-resistance of the sense transistor is 110 .OMEGA., and the wiring resistance of the sense transistor is 40 m.OMEGA., the wiring resistance ratio in the power transistor circuit is 1000 times higher than that in the sense transistor circuit.
Usually, a metal such as aluminum is used as a wiring material. The temperature coefficient of aluminum is about 3000 ppm/T and that of a MOS transistor is about 4500 ppm/T. Therefore, the current mirror ratio, which is 1000 in the case exemplified above, fluctuates according to temperature. An example of the fluctuation percentages is shown in FIG. 5. If the current mirror ratio varies according to temperature, current detection accuracy decreases.